Needleless syringe devices are known from WO 94/24263. In this document, a needleless syringe is disclosed which entrains particles in a gas stream accelerated through a nozzle so that the particles may be injected into a target, such as human skin or other cells. For many applications, there is a need for the particles to be maintained in a sterile environment prior to actuation of the device. WO 94/24263 discloses for this purpose a particle cassette comprising a central annular ring having rupturable diaphragms sealed to each face so as to form a self contained sealed unit containing the particles to be injected. Upon actuation of the device, the diaphragms rupture allowing the particles initially contained between the diaphragms to be entrained in the gas flow and then delivered into the target.